The gates wont shoot you at all if you stay at a range of 9 to 10 km from it. I have taken out entire gates and not taken a single hit from them. As for the spheres and cubes. Kill them first or have team mates keep them busy while you kill the gate. Another hint for you don't go at it straight on, but from an angle so your front and aft weapons can both fire on it at the same time.